Something is way off here, let's see if we can figure it out. First, do you have nitrate in the tap water on its own?
The reason nitrate is low (or should be) in aquatic plant tanks is because the plants prefer ammonia/ammonium, and they literally grab all they can. And they are faster at this than the nitrifying bacteria. That means very little ammonia/ammonium gets taken up by the nitrifiers, which in turn means lower nitrite and then lower nitrate, since the plant's use of ammonia/ammonium does not produce nitrite or nitrate. Second, theytwill not turn to nitrate unless forced, because it takes the plant about 24 hours to "change gears," and when they do, they then have to expend considerable energy converting the nitrate back into ammonium. So it is a "last resort"measure. There is also, according to a study cited in Walstad, evidence that plants would prefer nitrite over nitrate, but for the same reasons they do not do this unless they have no other options. Most of us with low-tech or natural planted tanks will never see plants turning to nitrite/nitrate because the ammonia is so plentiful it is hardly going to run out without other nutrients (carbon especially) and light not being adequate anyway.
Nitrate is poisonous to fish, as is ammonia and nitrite, but it acts differently. I had a good discussion with Neale Monks about this, and he said the thinking now is that nitrate slowly weakens the fish, making it more susceptible to other things such as disease. And stress, which is behind 90% of all aquarium fish disease. Levels vary depending upon the fish species and the exposure time, but he advises that it should never be allowed to rise above 20 ppm, and there are species that will have serious issues with even this--cichlids are known to be highly sensitive to nitrate, and on the cichlid sites they are now advising that diseases like hole-in-the-head is more likely nitrate-related than previously thought. Malawi bloat has also been linked to nitrate.
If you wish, we can look at issues causing this and suggest possible corrective measure, once we know the tap water question.